A multi-fuel engine is an engine designed to combust multiple types of fuel with air in its operation. Multi-fuel engines may be desirable where cheaper and cleaner fuel sources, such as natural gas, are available as a primary fuel source but a secondary fuel (e.g., diesel) is desired for performance reasons or as a backup in the event of depletion of the primary fuel source. A dual fuel engine, for example, may be designed to run on either or both of diesel and natural gas as fuel at a range of relative ratios depending on performance requirements and availability of the fuel sources. In one implementation of a dual fuel engine, the engine may primarily run on natural gas with the diesel being used as an ignition source.
The relative air-fuel ratio, or lambda (λ), in a multi-fuel engine may be defined as the ratio of the actual air-to-fuel ratio (e.g., mass of air/total mass of fuel), AFR, to the stoichiometric air-fuel ratio, AFRSTOICH, which is the air-to-fuel ratio at stoichiometry where just enough air molecules are present to achieve complete combustion of all of the fuel molecules. In fuel rich mixtures (excess fuel), λ is less than one, while in fuel lean mixtures (excess air), λ is greater than one. In order to provide compliant emissions as well as to regulate engine performance, λ in multi-fuel engines may be tightly controlled. However, in engines using natural gas as a fuel source, accurate control of λ may be a challenge as the composition of natural gas may vary or may change with time. In particular, natural gas may include various combustible species such as methane (CH4), ethane (C2H6), and propane (C3H8), as well as various gases such as nitrogen (N2) and carbon dioxide (CO2) that do not participate in the combustion process. For some applications, it may be cost-prohibitive to employ analytical techniques to determine the exact composition of the natural gas used in the engine.
U.S. Pat. No. 8,733,298 describes a method for determining a blend ratio of petrodiesel fuel and biodiesel fuel in a multi-fuel compression ignition engine. Specifically, the method uses an in-cylinder pressure sensor (ICPS) to calculate the heat released in the cylinder combined with the injected fuel quantity to estimate a lower heating value (LHV) for the fuel, which is then correlated with a percent biodiesel in the petrodiesel/biodiesel fuel mixture. While effective, further improvements that address multi-component fuels having compositions that vary with time, such as natural gas, are still wanting.
Clearly, there is a need for improved methods and systems capable of controlling air-fuel ratios in engines operating with a fuel source having an unknown composition.